To identify and characterize lymphocyte cell surface determinants, especially those which mark functional lymphocyte subpopulations; and to functionally characterize lymphocyte subpopulations carrying different surface markers. To genetically map loci controlling lymphocyte surface determinants. To fuse myeloma cells with normal antibody producing cells, in order to obtain B cell hybrid culture lines which secrete large quantities of antibodies of interest or utility. a) To establish the distribution and determine the functional role of the various immunoglobulin surface determinants on B cells, particularly the recently defined IgD. b) To study the ontogenetic mechanism of allelic (haplotype) exclusion which occurs during B cell differentiation. To fuse myeloma cells with normal antibody producing cells, in order to obtain B cell hybrid culture lines which secrete large quantities of antibodies of interest or utility. To study the mechanism and functional significance of the selective expression of loci in the I-region of the major mouse histocompatibility complex (H-2) which control determinants on functional T cell subpopulations. To study the genetics, immune mechanisms and relationships between the occurrence of reticulum cell sarcomas and abnormalities in regulation of the immune response in SJL/J mice.